Online Drought Indicators and Indices

Origins: Developed in China, CZI builds on the ease of calculation provided by SPI and improves on it by making the calculations even easier for the user. A statistical Z-score can be used to identify and monitor drought periods. The index was first used and developed in 1995 by the National Climate Centre of China.

Characteristics: CZI is similar to SPI because precipitation is used to determine wet and dry periods, assuming that the precipitation obeys a Pearson type III distribution. It uses monthly time steps from 1 to 72 months, giving it the ability to identify droughts of various durations.

Input parameters: Monthly precipitation.

Applications: Similar to SPI, in which both wet and dry events can be monitored over multiple timescales.

Strengths: Simple calculations, which can be computed for several time steps. Can be used for both wet and dry events. Allows for missing data, similar to SPI.

Weaknesses: The Z-score data do not require adjustment by fitting them to gamma or Pearson type II distributions, and it is speculated that because of this, shorter timescales may be less well represented compared with SPI.

Resources: All calculations and explanations of the CZI can be found in the literature.

References:
Edwards, D.C. and T.B. McKee, 1997: Characteristics of 20th century drought in the United States at multiple time scales. Atmospheric Science 634: 1–30. (For more information on this paper, please contact the IDMP HelpDesk).